So I was very curious to what the differences were to the different combinations, so I dug through my cotton scrap bin and luckily there was a bunch of thrips that were the same size, 3 1/2" so I went with it and did a sample of them, Now to bad yo can touch my sample, it would give you an idea on what combination would work best for the summer quilt, I am very happy with what I learned, I did take pictures of my sample, but it mostly looks the same, I will try to describe what I feel etc.
The first rows were just cotton with cotton, not a bad feel, the seam isn't to bad, but not really any body. You would have to be careful what colors are back to back as the light is showing the dark on the back side.
second set had a third piece of cotton, nice feel, still pretty light, seam isn't to bulky, you could definitely use a neutral so it wouldn't show through. The biggest thing is now you have to keep track of 6 layers compaired to the 4 so a little more difficult.
third set I added more cotton, not to impressed, the added layers are difficult to keep even, at least compared to the 4.
Then in the middle I added a single layer of flannelette. Worked well, I like the feel and the seam is fine. It adds some bulk and would probably make a better spring or fall quilt depending on your weather. The last set is probably my favorite, one of cotton and one of flannelette. Nice body, the seam isn't really bulky, gives the one side a nice soft feeling. light enough for a summer quilt still. I just used a lighter color but I imagine depending on the back you may have to watch as it may show through the cotton side.